A conventional electro-acoustic transducer having a vibrating function (hereinafter referred to as a transducer) is disclosed in Japanese Patent Laid-Open Application No. 2000-153231. This conventional transducer is described referring to FIGS. 5A and 5B. FIG. 5A is a plan view and FIG. 5B is a cross sectional view.
Referring to FIGS. 5A and 5B, the transducer's voice coil 10a is fixed to a diaphragm 10. A magnetic circuit 11 comprises a magnetic circuit portion 11a which generates a driving power by flowing an electric current in voice coil 10a, and a weight portion 11b which is integrated with the magnetic circuit portion 11a. The weight portion 11b is added for a purpose of sensing vibration of vibration section 13, which will be referred to later. If the vibration section 13 generates sufficient vibration, the weight portion 11b can be omitted.
Magnetic circuit portion 11a and weight portion 11b are supported by a frame 16 via a suspension 12. Vibration section 13 comprises magnetic circuit 11 and suspension 12. Diaphragm 10 and voice coil 10a constitute a mechanical resonance circuit of an acoustic section. Magnetic circuit 11 and suspension 12 constitute a mechanical resonance circuit of vibration section 13.
Weight portion 11b is a molded resin containing tantalum powder, and suspension 12 and magnetic circuit portion 11a are integrated with the weight portion 11b through an insert molding process to provide a one-piece component. A baffle 17 is bonded to a periphery of diaphragm 10, and attached to frame 16.
Now, operation of the above-configured electro-acoustic transducer having a vibrating function is described below.
As voice coil 10a is disposed in a magnetic gap A of magnetic circuit portion 11a, when an AC current is applied, voice coil 10a generates a driving force. Since a weight of voice coil 10a is very small relative to that of magnetic circuit 11, magnetic circuit 11 does not vibrate at most frequency ranges, while voice coil 10a alone vibrates. Thus, diaphragm 10 is vibrated by voice coil 10a to generate sounds at most frequency ranges.
Since vibration section 13 is for sensing vibration of a human body, a mechanical resonance frequency of vibration section 13 is set at a certain frequency that is lower than that of the acoustic section. Mechanical impedance of vibration section 13 becomes smallest at the mechanical resonance frequency. Therefore, even with a small driving force, vibration section 13 can generate a vibration large enough to be sensed by the human body. Vibration force at this time is determined by a product of vibration section 13's weight (that is a weight of magnetic circuit 11, in an approximation) and acceleration of vibration section 13.
In the conventional transducer having a vibration function operating in accordance with the above-described principle, the mechanical resonance circuit comes to have a high resonance sharpness Q in order to vibrate vibration section 13 which has a large mass. As a result, vibration section 13's mechanical resonance frequency disperses largely relative to resonance frequency signals delivered to voice coil 10a from outside for vibrating vibration section 13. This dispersion leads to problematical dispersion of vibrating force. Dispersion in mechanical resonance frequency is caused by weight dispersion of vibration section 13, dispersion in material thickness, width, Young's modulus, and the like of suspension 12, and supporting position dispersion of suspension 12 and other factors.
The present invention addresses the above problems and provides an electro-acoustic transducer having a vibrating function, wherein the mechanical resonance frequency of the vibration section can be adjusted at low cost, and dispersion of vibrating force is reduced.